1. Field of the Invention
The present invention relates to a multi-central processing unit system (multi-CPU), more particularly a multi-CPU system provided with a fault monitoring facility for monitoring faults in the system.
A multi-CPU system to which the present invention is applied, is comprised of a plurality of central processing units connected through a system bus to each other. The system sends and receives data between these central processing units through a plurality of bus interface units inserted in the system bus.
The system bus for the above-mentioned multi-CPU system is known as a "TOX-BUS" and enables an extremely high speed and efficient bus transfer. For example, the system bus is effective for a base system in a duplex configured electronic exchange.
Such electronic exchanges require a particularly high reliability. Accordingly, it is necessary that some sort of fault monitoring facility be provided for the multi-CPU system of that base system.
2. Description of the Related Art
as will be explained in detail later referring to the drawings, according to a conventional fault search sequence carried out in a multi-CPU system, one of the central processing units in the system accesses a first bus interface unit (BIU) and reads out and analyzes an error status and then accesses a second bus interface unit and reads out and analyzes the error state of the second bus interface unit. This operation is repeated until the faulty portion of the system is reached. Therefore, there was the first problem that it took a massive amount of time to find the fault and the time until restoration of the system was considerably prolonged.
Further, since it was not possible to access units ahead of (downstream side) the faulty portion, there was the second problem that the bus interface units, central processing units, and other functional units ahead of the faulty portion could not be notified of the occurrence of the fault. If it were possible for other central processing units to be notified of the occurrence of a fault, then the other central processing units could operate and deal with the faulty situation in the multi-CPU system.